The acquisition of stimulus control over conditioned behavior will be studied by the following three approaches: 1. What types of behavior are established during S minus ? How is this behavior related to the behavior that is reinforced during S plus ? To what extend does S minus (as opposed to the absence of S plus) establish control over "non-positive" responding? 2. The study of the relationship between various by-products of discrimination learning (e.g., behavioral contrast, the peak shift, inhibitory stimulus control, an aversive function of S minus and emotional behavior in the presence of S minus), will be continued with particular emphasis on how S plus and S minus interact following a single exposure to S minus. 3. The acquisition of an autoshaped response will be studied in the pigeon, rat and the goldfish so as to determine the influence of the following parameters: duration of intertrial interval, relative duration of intertrial interval, probability of reinforcement and the occurrence of a second stimulus during the intertrial interval.